The purpose of the study is to examine the effects of family stress and family routines on parents' perceptions of infant behavior and on the risk of illness in 300 infants aged four to twelve months. Information on sources of family stress and routines will be repeatedly obtained via questionnaires at scheduled well-baby checks during the first year of life. Questionnaire measures of infant temperament and development will be obtained at the same time. In addition to a life events checklist, family stress will be indicated by the frequency of minor stressors termed "hassles", as measured by the Family Hassles Questionnaire. The extent of routines such as mealtimes shared by family members is conceptualized as a potential source of social support, and measures of social network size and actual support will also be obtained. Thus, the study will attempt to explore the effects of daily family life, in addition to the effects of dramatic life changes. Families in the study will be selected from participants in a large, ongoing epidemiological study of lower respiratory infections in infants. This epidemiological study will provide data on the occurrence of all illnesses and injuries occurring to infants in the proposed study during the year they are observed. Data on demographic characteristics of the families and other risk factors for illness will also be available from this respiratory study. It is anticipated that the proposed study will yield new information about the effects of family life on the health and well-being of preverbal infants, and that the study will contribute to our understanding of the role of psychosocial factors in the etiology of illness in infancy.